Presently cationic dyes are used primarily for the dyeing of polyacylonitrile (hereafter, acrylic) fabrics. Some polyester and polyamides that have been modified to carry acidic groups at the molecular level, usually sulfonic acid groups, are also dyed with cationic dyes. Generally cationic dyes are of outstanding brightness of color and many modern cationic dyes have good fastness properties.
Extensive literature is to be found that describes cationic dyes and their application. The Color Index, a joint publication of The American Association of Textile Colorists and Chemists, of Research Triangle Park, N.C., and The Society of Dyers and Colorists of Yorkshire, England, lists current and past cationic dyes and describes their application. The Color Index uses the term, basic dye, which is an older name for cationic dyes.
The Color Index also gives names of dyes by application class; which names are used world wide. Below we shall use the names assigned by The Color Index where applicable, thus a name, such as Disperse Red 1, denotes a particular dye listed in The Color Index under disperse dyes. The Color Index gives application information regarding the dye in the appropriate section and may give the chemical formula of the dye, if such information is available, in a separate section.
Perhaps less well known is that there are many dyes, for example, many disperse or solvent dyes, that are not normally cationic, but that become positively charged and thus cationic in an acidic solution. For purposes of this invention, we shall include such dyes in our definition of cationic dyes, provided they are in a solution that causes a significant portion of their molecules to carry a positive charge.
There are a number of fibers from animal origins that include silk, wool, alpaca, leather, and so forth that may readily be dyed with cationic dyes. Presently such dyeings are not fast to washing so that fixing agents, such as tannic acid (or various synthetic materials with comparable properties) followed by metal salts, such as tarter emetic, are used. Such dyeings lack light fastness, and, with current fears of chemicals in general, lack environmental acceptance.
Some fibers are produced from plastics that are polymerized with some portion of monomers that carry sulfonic acid groups as part of their molecular structure. Such fibers are readily dyeable with cationic dyes. Polyester, polyamides, polyacrylonitrile, polyethylene, and polypropylene all have been made, at least in laboratory quantities, to incorporate such groups. DuPont Company of Wilmington, Del., for example, has a series of polyester products that contain such acidic groups. These materials are all dyeable using cationic dyes.
With the exception of cellulose based materials such as cotton, linen, or ramie, fibers that are not cationically dyeable are usually hydrophobic and dyed with disperse dyes. Polyester, polyolefins, and some acrylics are such fibers.
Cellulose based fibers, generally speaking, have only a weak affinity for cationic dyes. As is described below, cellulose materials may be treated with various agents that increase the affinity of the fiber for cationic dyes.